[dd4hep] align runtime env with lcg release (#18082)

var/spack/repos/builtin/packages/dd4hep/package.py

@@ -72,6 +72,8 @@ def cmake_args(self):
     def setup_run_environment(self, env):
         # used p.ex. in ddsim to find DDDetectors dir
         env.set("DD4hepINSTALL", self.prefix)
+        env.set("DD4hep_DIR", self.prefix)
+        env.set("DD4hep_ROOT", self.prefix)
 
     def url_for_version(self, version):
         # dd4hep releases are dashes and padded with a leading zero